[PDF] Dynamic positioning system design for “Blue Lady”. Simulation tests

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POLISH MARITIME RESEARCH, Special Issue 2012/S1

Dynamic positioning system design for "Blue Lady". Simulation tests

ABSTRACT

The dynamical positioning system is a complex control consisting of a number of components, including: filters, observers, controllers, and propeller allocation systems. The design and preliminary analysis of operational quality of system operation are usually done based on numerical simulations performed with the aid of the mathematical model of the ship. The article presents a concept of the dynamic positioning system applied to steering the training ship Blue Lady used for training captains in the ship handling research and training centre owned by the Foundation for Safety of Navigation and Environment Protection in Ilawa/Kamionka. The simulation tests performed in the numerical environment

of Matlab/Simulink have proved the usability of the designed system for steering a ship at low speed.

Key words: dynamic positioning system; marine systems; ship control

INTRODUCTION

Automatic control of ships has been studied for over a century. In 1911 Elmer Sperry constructed the first automatic ship steering mechanism called "Metal Mike". Today, the range of marine vessels covers a huge diversity of vehicles such as remotely operated vehicle (ROVs) and semi-submersible rigs. Automatic control systems for heading and depth control, way- point tracking control, fin and rudder-roll damping, dynamic positioning (DP), thruster assisted position mooring (PM) etc. are commercial products [1]. DP systems have traditionally been a low-speed application, where the basic DP functionality is either to keep a fixed position and heading or to move slowly from one location to another. In addition specialized tracking functions for cable and pipe layers, and remote operated vehicle operations have been available [2]. The dynamic positioning system for marine vessels has been divided into a set of dedicated modules with designed tasks. The most important components are shown in Fig. 1 [1]. The guidance system is used for planning the route for ship motion from the starting point to a selected target point. In the DP system the guidance system generates a smooth trajectory with position coordinates and course which lead to the next position. The unit responsible for signal processing monitors the measured signals and performs quality tests to detect extensively large variations, wild points, frozen signals, and/or signal drifts. Erroneous signals are to be detected and not used for further operations. The signal processor should check and

evaluate the signal based on the tests of individual sensors when the measurements done on redundant sensors are available.

A typical marine vessel equipped with a DP system has two or three gyrocompasses and the same number of position calculation systems. The main task of the observer is to provide low-frequency estimates of the vessel"s position, heading and velocity. Rapid oscillating movements caused by waves are to be filtered out. The observer should also be able to predict ship movements in the situation when ship heading and position measurements become unavailable (dead reckoning). In low-speed applications the controller calculates three desired parameters: the surge force, the sway force, and the yaw moment. Depending on the performed operation modes, the controller takes into account the system state estimates, the reference trajectory and the measured environmental conditions in the calculations.

Fig. 1. The major components

of a positioning control system for marine vehicles The internal logic of the controller controls the modes of switching between different operation types. The thrust allocation system maps the desired forces and yaw moment obtained from the controller into the required

Mirosław Tomera, Ph. D.,

Gdynia Maritime University

Dynamic positioning system design

for "Blue Lady". Simulation tests POLISH MARITIME RESEARCH Special Issue S1 (74) 2012 Vol 19; pp. 57-65

10.2478/v10012-012-0024-4

58POLISH MARITIME RESEARCH, Special Issue 2012/S1

Dynamic positioning system design for "Blue Lady". Simulation tests propeller settings, such as the propeller rotational speed and pitch ratio, the angles of the rudder blade and azimuth thrusters. It is important that these set-points are done in an optimal manner, which most frequently leads to minimisation of energy consumption. The number of marine vessels with the installed DP systems is continuously growing due to deep-sea gas and oil mining. At present, the DP systems are most frequently used on shuttle tankers which provide services for drilling rigs. The beginning of the dynamic positioning systems goes as far to the past as to the last century"s sixties when the first systems acting on horizontal plane in three directions of ship motion: surge, sway and yaw were introduced. These systems made use of one-dimensional single input/single output (SISO) algorithms of the PID controller, along with low-pass or notch filters. The description of the DP systems which includes early stages of their development was given by Fay [3]. In the last century"s seventies, more advanced ship steering methods were introduced which based on multidimensional optimal control and the Kalman filter theory. The first solution of this type was presented by Balchen, Jenssen and Saelid [4] as well as by Balchen, Jenssen, Mathiasen and Saelid [5]. The verification of this solution on a marine ship was later presented by Balchen, Jenssen and Saelid [6]. Later on, this solution was the object of further modifications and extensions, done by Saelid, Jenssen and Balchen [7] who proposed a new algorithm of adaptation to changing frequency in order to improve the quality of system operation within a wide range of changes of environmental conditions. Grimble, Patton and Wise [8] presented an extended the analysis of the Kalman filter, comparing it to a notch filter which was earlier used in the dynamic positioning systems. Then, Fung and Grimble [9] proposed a self-tuning algorithm for automatic tuning of the Kalman filter matrix, obtaining good results. Despite the improvement in the filtration and estimation algorithms, the operation of the controller still based on the optimal control theory. However, some problems are observed in cases when linear PD controllers are used in the DP systems. Tuning the gain is a very complicated task which requires time-consuming tests done on sea with the DP system switched on. Unfortunately, the operational quality of the controllers changes following the changes of the level of environmental disturbances and load conditions. The DP operator has to tune manually the controller"s gain to adapt to changes of the environmental conditions. Another important issue is the robustness of the controller. The mathematical model used for modelling the ship motion on the sea, where the ship is subject to the action of wind, sea currents and waves, is strongly nonlinear and some phenomena are extremely difficult for mathematical modelling. The design of the DP controller has to take into account its expected robustness. Since last century"s nineties, this issue has been taken into account in designing linear DP systems by using the H technique. Designs of this type can be found in publications by Katebi, Grimble and Zhang [10], Kijima, Murata and Furukawa [11], Nakamura and Kajiwara [12], Tannuri and Donha [13], Donha and Tannuri [14] as well as Gierusz [15]. This type or controller meets well the robustness requirements in the presence of large changes of environmental conditions. But it is still the linear controller which bases on the linear model of the object; therefore different controllers should be designed for a number of operating points defined in the space of states in the vicinity of the point which the ship reaches during the executed operation. In order to avoid problems connected with linearization in the DP systems, nonlinear controllers have been introduced. Fuzzy controllers were proposed by Stephens, Burnham and Reeve [16], Broel-Plater [17], as well as by Chang, Chen and Yeh [18]. Nonlinear controllers designed using the backstepping method and proposed by Aarset, Strand and Fossen [19], Strand and Fossen [20], Fossen and Grovlen [21] as well as by Bertin, Bittani, Meroni and Savaresi [22] were also successfully used. The publications by Fossen and Strand [23], Strand and Fossen [24] as well as Strand [25] present important issues of passive nonlinear observers with adaptive wave filtration. An advantage of the use of the nonlinear theory of passiveness was the reduced complexity of the code packages used for steering. Pettersen and Fossen [26], Pettersen, Mazenc and Nijmeijer [27] as well as Bertin, Bittani, Meroni, and Savaresi [22] have worked out the DP control algorithms for seagoing vessels in which the number of propellers is smaller than the number of freedom degrees (so-called under-actuated vessels). Agostinho, Moratelli, Tannuri and Morishita [28] as well as Tannuri, Agostinho, Morishita and Moratelli [29] have proposed the use of nonlinear sliding control in the DP system. The application of hybrid control theory proposed by Hespanha [30], Hespanha and Morse [31], Hespanha, Morse and Liberzon [32], as well as the fault-tolerant control proposed by Blanke, Kinnaert, Lunze and Staroswiecki [33] has made it possible to design a correct control architecture for integrating multifunction controllers which link discrete events and continuous steering. The effect of operation of such a controller applied in DP systems was described by Sorensen, Quek and Nguen [34], Nguyen [35], Nguyen and Sorensen [36] who proposed a design of the controller with the superior switching logic to select a proper controller and observer from a set of controller and observers assumed for different environmental conditions. The applicability of the DP systems on shuttle ships, where such operations as position keeping, sailing along a given trajectory, and unloading with FPSO (floating production storage and offloading) are performed, was analysed by Morishita and Cornet [37], Morishita, Tannuri and Bravin [38], Tannuri and Morishita [39], as well as Tannuri, Saad and

Morishita [40].

The article presents a design of the DP system applied to steering the training ship Blue Lady. The investigations performed on the observers which could be hypothetically used in the designed DP system had been done earlier and were presented in the following articles and papers: the discrete Kalman filter [41], the continuous Kalman-Bucy filter [42], and the extended Kalman filter and nonlinear observer [43]. THE MATHEMATICAL MODEL OF BLUE LADY DYNAMICS FOR LOW SPEED The training ship Blue Lady is owned by the Foundation for Safety of Navigation and Environment Protection in Ilawa and is used for training captains to perform complex and difficult manoeuvres of a large ship. The ship, made of epoxide laminate in the scale of 1:70, is a replica of a tanker used for transporting crude oil. The overall length and breadth of the physical model are, respectively, L OA = 13.75 [m], and B = 2.38 m. In the full- load state its mass is m = 22830 [kg], moment of inertia I z

436830.3 [kgm

2 ], while center of gravity is located at midship, hence x G = 0 [m]. The model is equipped with a set of actuators with electric motors fed from an accumulator battery, including the main propeller with a conventional plane rudder, and four jet thrusters: two tunnel (the bow thruster and the stern thruster) 59
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